The present invention relates to the production of a polycrystalline body comprised of a mass of diamond and/or cubic boron nitride crystals bonded together by a medium comprised of silicon carbide and elemental silicon, and more particularly to a method for improving the density and degree of bonding of such composites.
While a wide variety of techniques have been proposed for forming said polycrystalline bonded composite bodies, a preferred method for fabricating such bodies is disclosed in commonly-assigned U.S. Pat. No. 4,220,455, the disclosure of which is expressly incorporated herein by reference. Briefly stated, such patent discloses a process which utilizes a partial vacuum for improving the infiltration of fluid silicon during the fabrication of such composite bodies. Somewhat similar processes are disclosed in U.S. Pat. Nos. 4,238,433 and 4,242,106. The improved silicon infiltration enables a denser, more completely bonded composite body to be fabricated than was feasible prior to the invention disclosed in such patent.
One drawback in the process of such patent became apparent when such process was practiced in the fabrication of bodies of large volume, e.g. volumes greater than about 10 mm.sup.3. The drawback noted was that porosity and unbonded surfaces in the bulk were discovered upon finish grinding of the composite bodies which conditions resulted in a composite body which was unacceptable for many intended applications, such as wear surfaces and cutting tools. The porosity was reduced by retarding the heating rate to infiltration temperature to about 10.degree. C. per minute or slower; however, the described conditions still remained to such a degree that the resulting composite body still was unacceptable for some intended applications. It is opined that volatile impurities in the materials of fabrication do not completely volatilize or, if volatilized during the infiltration process, are unable to escape from the body, thus resulting in porosity and unbonded surfaces in the bulk. While the slower heating rates to infiltration temperature apparently permitted additional quantities of such impurities to escape from the bulk, sufficient impurities apparently remain in the body. Further, dimensional tolerances without use of fixed boundaries on all surfaces of the composite during fabrication were found not to be possible due to the unacceptable level of porosity and unbonded surfaces in the bulk.
The present invention solves the problems of porosity and unbonded surfaces in the body of the composite bulk by employment of a vacuum firing step which will be disclosed in detail herein.